United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #: 9809240136


September 21, 1996

Document Control Desk
ATTN: Chief, Planning, Program and Management Support Branch
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001

Interim Report of Evaluation of a Deviation Pursuant to 10 CFR 21.21(a)(2)

Ref.: 1. Letter, J. F. Mallay (SPC) to Document Control (NRC), "Interim Report of Evaluation of a Deviation Pursuant to 10 CFR 21.21(a)(2)", NRC:98:029, May 11, 1998

The following interim report was provided by the reference letter pursuant to the requirements of 10 CFR 21 to submit an interim report on issues that will not be completed within 60 days of discovery.

Interim Report No. 98-002 "TOODEE2 Axial Nodalization"

The Part 21 evaluation related to this interim report has been completed. The conclusion of the evaluation was that the deviation described in the interim report did not constitute a reportable defect per 10 CFR 21. A description of the deviation reported in Reference 1 along with a discussion of the actions that SPC has taken is provided in the attachment to this letter.

Those SPC customers potentially affected by this issue will be provided a copy of this letter.

If you have any questions or if I can be of further assistance, please call me at (509)375-8757.

Very truly yours,

James F. Mallay, Director
Regulatory Affairs


cc: Mr. E. Y. Wang (USNRC)
Mr. R. Caruso (USNRC)
Project No. 702

Siemens Power Corporation
Nuclear Division [Illegible Address] Tel: (509) 375-8100
Engineering & Manufacturing Fax: (509) 375-8402

Closure of Interim Report (98-002)

Subject: Closure of an interim report of evaluation of a deviation pursuant to 10CFR 21.21(a)(2)
Title: TOODEE2 Axial Nodalization
Identification of Basic Activity: PWR Large Break LOCA Analysis
Basic Activity Supplied by: Siemens Power Corporation - Nuclear Division
Nature of Deviation: The axial nodalization in TOODEE2 (part of the NRC-approved EXEM/PWR LBLOCA evaluation model) consists of 3 inch nodes near the peak power node, where rupture and peak cladding temperature are expected to occur, and larger nodes in other regions. The methodology requires that rupture be calculated to occur in a 3 inch node, and standard practice at SPC has been that PCT should also be calculated within a 3 inch node. Larger axial node sizes of between 6 inches and one foot are commonly used at the top of the core where the power is lower than the axial peak value. With this nodalization, the peak cladding temperature for an axial power profile skewed toward the top of the core generally occurs at or slightly above the peak power node which is between 9 and 10.5 feet for a 12 foot core. If the larger nodes at the top of the core are replaced by 3 inch nodes, the EXEM/PWR model may calculate an even higher peak cladding temperature near the top of the fuel (above 10.5 feet) even though power is somewhat lower at this elevation.
The prediction of the peak cladding temperature at reduced power nodes near the top of the core is unrealistic. Reflood cooling is by steam and entrained liquid flowing from the quench front in the core. Once this mechanism is established and is calculated to be sufficient to cool the maximum power nodes and the high power nodes immediately downstream, higher elevations in the core, which are at reduced power, will also be cooled.
The unrealistic peak cladding temperatures that are calculated to occur at or very near the top of the core are due to the assumption of significantly degraded and delayed heat transfer at the top of the core relative to the middle of the core. Investigation of the source of this degraded and delayed heat transfer involved examination of the FCTF heat transfer correlations. These correlations fit data obtained from FCTF tests using heaters with a center-peaked "cosine type" power profile. For this power profile, the observed heat transfer at the top of the core appears significantly delayed and reduced compared to lower elevations. Phenomenologically the entrained flow that cools the core during reflood has been established at lower elevations in the core, but it exists also at the higher elevations. In flowing through the central high-power high-temperature core regions, however, the steam portion of the flow becomes superheated. With a cosine axial power profile, the top of the core is a low-power low-temperature region, and the superheated steam temperature exceeds the cladding temperature for some period of time during reflood. In reducing the FCTF data, this effect appears as a delayed and degraded heat transfer at the upper core elevations. The degraded heat transfer may reflect real conditions for a cosine power profile but does not present a PCT problem because of the low power and low temperatures in this region.
In SPC's methodology, the calculated heat transfer behavior as a function of elevation for the cosine power profile is applied to skewed profiles using a "Z-equivalent" approach. In this approach the axial power profile is integrated to trip elevation of interest, and an equivalent elevation of the FCTF cosine having the same integral is determined. This equivalent elevation is then used in the cosine FCTF correlations to calculate the heat transfer coefficients that are applied at the skewed profile elevation. Using this methodology at the upper end of the core, the delayed and degraded cosine heat transfer characteristic of a low-power low-temperature region is applied to a skewed power profile which is neither low-power nor low-temperature at these elevations. The result is the calculation of a very conservative cladding temperature which is unrealistic.
The delayed and degraded cosine heat transfer is inherent in the FCTF data, and since the FCTF correlations are a fit to these data, the effect is imbedded in the correlations. However, the degradation is also enhanced by an additional conservatism forced on the correlations to ensure their conservatism relative to the FCTF data base. This conservatism is in the form of an additional time delay, or offset time, which further delays the initiation of reflood heat transfer as calculated by the FCTF correlations.
Since SPC does riot have any results from skewed power profile tests, outside available sources were searched. It was found that the USNRC had sponsored FLECHT experiments with low flooding rates and skewed axial power profiles reported in WCAP-9108. This test series included 48 tests with a top skewed power profile. After carefully examining the test data, it is clear that the delayed heat transfer developed from the "cosine profile" is not representative of skewed power profile data and is very conservative when applied to skewed power profiles. No significant delay time in developing heat transfer is observed within one foot of the peak power location for all 48 tests of the skewed power profile tests.
In order to justify a change in the FCTF correlations at the top of the core, additional tests would be required for axial power profiles skewed toward the top of the core. It is not feasible to conduct these test in a reasonable time period to support a change to the methodology to correct for this conservatism.
Results obtained using this cosine-based model are always conservative. Even though the effect is unrealistic for highly skewed axial power profiles, the results can be used to show conformance to 10 CFR 50.46 criteria. To assure that a bounding PCT is calculated increased nodalization will be used at the top of the core.

Corrective Actions:

The deviation was addressed by incorporating modifications to the EXEM/PWR ECCS Evaluation Model as part of the development of the SEM/PWR-98 ECCS Evaluation Model, Reference 1. The modification consisted of a requirement for increased nodalization at the top of the core in the TOODEE2 heatup calculation. This modification will assure that the SEM/PWR ECCS Evaluation Model will always predict a conservative peak cladding temperature.


1. EMF-2087(P), "SEM/PWR-98 ECCS Evaluation Model for PWR LBLOCA Applications", Siemens Power Corporation, August 1998

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